There is no direct lifetime. This depends on many factors such as runtime, ambient temperatures and heat generation during operation. Depending on the use, the AIO can run for 3 years or even 8 years and longer. This cannot be determined exactly. If the cooling performance decreases, it is usually more likely due to a dried out thermal paste. I would replace it every 2 years. Otherwise, the AIO is only defective when the pump fails at some point, more can not be broken.